Salt is known to enhance the IFT reduction property of surfactants, however for some cases IFT increases in presence of salt. What are the possible reasons for this binary nature of salt in IFT reduction by surfactant ? How does the salt particles interact with the oil-water interface in presence of surfactant ?
Gibbs adsorption isotherm tells us that the IFT reduction is proportional to the molecules adsorbed at the interface. In the presence of surfactants, salt affects the IFT via its effect on the solubility of surfactants in water. Lowest IFT is obtained when the surfactant has "balanced" (equal) solubility in both phases. So if the surfactant is too soluble in water, adding salt will reduce the IFT by reducing the surfactant's solubility in water. If the surfactant is not soluble enough in water, adding salt will increase the IFT by reducing the surfactant's solubility in water. The salt ions have a negligible direct effect at the interface in the presence of surfactant.
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